A light emitting device using a semiconductor light emitting element has advantages that it is a small size, has good electrical efficiency and can emit a bright color, and there is no fear of burning out an electric bulb because the light source is a semiconductor element. Further, it has characteristics that it is excellent in initial driving property and tough in vibration and the repetition of on-off lighting.
Further, for the light emitting device using a semiconductor light emitting element, there is developed a light emitting device which has a light emitting color which is different from the light of the light emitting element by carrying out a wavelength conversion of the portion of the semiconductor light emitting element by a phosphor, and by mixing said wavelength-converted light and no wavelength-converted light to be emitted. Specifically, the light emitting device which emits light in a white color can be used in wide fields such as a general illumination, a display and a back light for liquid crystal, by the constitution.
In the white color light emitting device, a blue light emitting element using an InGaN-base material is used as a light emitting element, a YAG-base phosphor which is represented by the composition formula of (Y, Gd)3(Al, Ga)5O12 is used as a phosphor, and a white light emitting color is obtained by the theory of color mixture of light. Namely, after blue light which was discharged from a light emitting element is emitted in the phosphor layer, it is discharged outside after repetition of absorption and scattering in the layer. On the other hand, the blue light absorbed in the phosphor works as an excitation source and emits yellow fluorescence. The yellow light and the blue light are mixed and the mixture is recognized as a white color by human eyes.
However, the above-mentioned white color light emitting device has defects that there is little red component in luminescence, color temperature is high, and only illumination light having low color rendering property which is deficient in red is obtained.
Namely, a conventional light emitting device which emits white light can hardly obtain a long wavelength side luminance at a visible light region, therefore it has been a light emitting device which exhibits a slightly bluish white color. On the contrary, there has been strongly required a light emitting device which exhibits a slightly reddish warm color-base white color, in the illumination for a window display, illumination for a medical spot and the like. Further, there has been strongly required a light emitting device which exhibits a white color which is soft for human eyes and near to an electric bulb color, also for general illumination.
However, since a conventional phosphor emitting reddish light has low efficiency caused by near ultra violet to blue light excitation and low chemical and thermal stabilities, durability is not adequate, therefore it is not practically used. Further, human eyes feel that the wave length region of a reddish component is dark. Accordingly, it is required that a red color region has higher brightness so that eyes feel the same level brightness as a green region and a blue region.
Under these circumstances, in the pamphlet of International Open Patent No.01/40403, there is disclosed an MxSiyNz:Eu nitride phosphor (wherein M is at least one of alkali earth metals at least selected from a group of Ca, Sr, Ba and Zn. z=(2/3)x+(4/3)y) which increased a red component in comparison with a conventional phosphor.
However, the nitride phosphor disclosed in the pamphlet of International Open Patent No.01/40403 obtains a slightly reddish white color light, for example, by combination with a blue light emitting diode, but the improvement of brightness is further required.
Further, a conventional phosphor emitting red light has inadequate efficiency caused by near ultra violet to blue light excitation and inadequate durability, therefore it is not practically used.
Accordingly, it is the first purpose of the present invention to solve the above-mentioned problems and to provide a phosphor containing a comparatively much red component and having high light emitting efficiency, high brightness and further high durability; and a production process thereof.
Further, it is the second purpose of the present invention to provide a light emitting device which can emit reddish warm color-base white color light and can emit light having the color rendering property.